1. Field of the Invention
The present invention is related to a process and an apparatus for the production of nitrogen by cryogenic distillation.
2. Related Art
The production of nitrogen by cryogenic distillation is well known and is described in numerous patent publications (J53-122861; U.S. Pat. No. 5,144,809; U.S. Pat. No. 4,867,773; U.S. Pat. No. 5,385,024; U.S. Pat. No. 4,927,441; U.S. Pat. No. 4,848,996; U.S. Pat. No. 4,883,519; U.S. Pat. No. 4,872,893; U.S. Pat. No. 4,869,742; U.S. Pat. No. 5,711,167; U.S. Pat. No. 5,611,218; U.S. Pat. No. 5,582,034; U.S. Pat. No. 5,402,647; U.S. Pat. No. 4,883,519; U.S. Pat. No. 5,385,025, WO/PCT/IB96/00323), and "Production of Medium Pressure Nitrogen by Cryogenic Air Separation" Gas Separation & Purification, 1991 Vol. 5, December, pp. 203-209.
Over the years numerous efforts have been devoted to the improvement of this production technique to lower the nitrogen cost which consists mainly of the power consumption and the equipment cost. As a general rule, an efficient process usually requires an additional degree of complexity of the equipment and the resulting cost will be increased. Therefore there is a constant need to come up with an efficient and simple process to assure a good trade-off between power cost and equipment cost.
The new invention described below utilizes the dephlegmation technique in a sub-section of the process cycle to combine distillation column and heat exchanger into simple and compact plate-fin exchanger equipment. Significant cost reduction can be achieved and at the same time good efficiency of the overall process can be maintained.
Dephlegmation is used to promote simultaneous heat and mass transfers so that a heat exchange function and a distillation effect can be conducted simultaneously in a single heat exchanger. Reflux condensation is an application of dephlegmation where a gaseous mixture being separated by rectification is simultaneously heat exchanged with a fluid stream that is raised in temperature or is vaporized by the heat exchange and thereby condenses fluid being rectified to create a countercurrent reflux flow for the rectified stream. In similar fashion, stripping reboil is another aspect of dephlegmation where a liquid flowing down inside a heat exchanger exchanges heat with another stream resulting in a partial vaporization and a formation of a rising vapor. This rising vapor being in direct contact with the down flowing liquid provides the stripping effect.
Several dephlegmator processes in cryogenics are described in previous patents and text books:
U.S. Pat. No. 2,861,432; U.S. Pat. No. 2,963,872; U.S. Pat. No. 5,592,832; U.S. Pat. No. 5,694,790; U.S. Pat. No. 5,030,339; U.S. Pat. No. 5,144,809; U.S. Pat. No. 5,207,065; U.S. Pat. No. 5,410,855; U.S. Pat. No. 5,438,836; U.S. Pat. No. 5,592,832; U.S. Pat. No. 5,596,883; "The Physical Principles of Gas Liquefaction and Low Temperature Rectification" by Mansel Davies published 1949 pp. 137-139, "Zerlegung der Luft" by H. Hausen published 1957 p. 164 and "Separation of Gases" by Ruheman, 2.sup.nd Edition, pp. 70, 174, 279-83, 291, 292. PA1 a) feed air is compressed, purified to remove contaminants which freeze out at cryogenic temperatures and cooled; PA1 b) cooled, compressed air is introduced into a distillation column wherein it separates into a fluid enriched in oxygen and a fluid enriched in nitrogen; PA1 c) a first liquid enriched in oxygen is removed from the bottom of the column, expanded and sent to a stripping dephlegmator; PA1 d) removing a second liquid enriched in oxygen and a third stream from said stripping dephlegmator; PA1 e) at least partially vaporizing at least part of said second liquid in a vaporizer to produce a further stream; PA1 f) sending said nitrogen enriched fluid from the column to a rectifying dephlegmator to produce a nitrogen product and a liquid, said rectifying dephlegmator exchanging heat with said stripping dephlegmator; and PA1 g) returning at least part of said liquid to the column as reflux. PA1 sending at least part of said third stream back to the column; PA1 mixing said third stream with feed air; PA1 mixing said third stream with feed air upstream of said purification step; PA1 sending said second liquid to a separator and sending liquid constituting at least part of said second liquid from said separator to said vaporizer; PA1 sending fluid from said vaporizer to said separator; and PA1 removing gas from said separator and expanding said gas. PA1 a) feed air is compressed, purified to remove contaminants which freeze out at cryogenic temperatures and cooled; PA1 b) cooled compressed air is introduced into a distillation column wherein it separates into a fluid enriched in oxygen and a fluid enriched in nitrogen; PA1 c) a first liquid enriched in oxygen is removed from the bottom of the column expanded and sent to a stripping dephlegmator; PA1 d) removing a second liquid enriched in oxygen and a third stream from said stripping dephlegmator; PA1 e) at least partially vaporizing at least part of said second liquid in a vaporizer to produce a further stream; PA1 f) sending said nitrogen enriched fluid from the column to a condenser to produce a nitrogen product and a liquid, said condenser exchanging heat with said stripping dephlegmator; and PA1 g) returning at least part of said liquid to the column as reflux. PA1 sending at least part of said third stream back to the column; PA1 mixing said third stream with feed air; PA1 mixing said third stream with feed air upstream of said purification step; PA1 sending said second liquid to a separator and sending liquid constituting at least part of said second liquid from said separator to said vaporizer; PA1 sending fluid from said vaporizer to said separator; and PA1 removing gas from said separator and expanding said gas. PA1 a) a distillation column; PA1 b) a heat exchanger; PA1 c) means for compressing feed air and sending said feed air to said heat exchanger and subsequently to said column; PA1 d) means for removing a first oxygen-enriched liquid from the bottom of said column; PA1 e) a stripping dephlegmator; PA1 f) a rectifying dephlegmator in thermal connection with said stripping dephlegmator; PA1 g) a vaporizer in thermal connection with said rectifying dephlegmator; PA1 h) means for sending said first liquid to said stripping dephlegmator; PA1 i) means for removing a second oxygen enriched liquid and a third gas from said stripping dephlegmator; PA1 j) means for sending at least part of said second oxygen enriched liquid to said vaporizer; PA1 k) means for removing a fluid from said vaporizer; PA1 l) means for sending a nitrogen enriched gas to said rectifying dephlegmator; and PA1 m) means for sending a liquid from said rectifying dephlegmator to said column and means for removing a nitrogen enriched product gas from said rectifying dephlegmator. PA1 a) a distillation column; PA1 b) a heat exchanger; PA1 c) means for compressing feed air and sending said feed air to said heat exchanger and subsequently to said column; PA1 d) means for removing a first oxygen-enriched liquid from the bottom of said column; PA1 e) a stripping dephlegmator; PA1 f) a condenser in thermal connection with said stripping dephlegmator; PA1 g) a vaporizer in thermal connection with said condenser; PA1 h) means for sending said first liquid to said stripping dephlegmator; PA1 i) means for removing a second oxygen enriched liquid and a third gas from said stripping dephlegmator; PA1 j) means for sending at least part of said second oxygen enriched liquid to said vaporizer; PA1 k) means for removing a fluid from said vaporizer; PA1 l) means for sending a nitrogen enriched gas to said condenser; and PA1 m) means for sending a liquid from said condenser to said column and means for removing a nitrogen enriched product gas from said condenser.
The above publications address the application of dephlegmators in the production of oxygen, nitrogen, hydrogen, helium etc.
Nitrogen is widely used in the industry for inerting, blanketing, ammonia production and electronics. The required purity of nitrogen is usually in the ppm's of oxygen for most applications and in the sub-ppb's for electronics. In some cases lower purity (1% to 2% O.sub.2 or 99% to 98% nitrogen) can be used.
The basic process for nitrogen production is shown in FIG. 1. This process is also called the classical process.
Air is compressed in a main air compressor 1 and then purified in 3 to remove water and carbon dioxide. It is cooled in heat exchanger 5 and sent to the bottom of column 9 where it separates into an oxygen enriched bottom fraction 12 and a nitrogen enriched top fraction. Part of the nitrogen enriched fraction is removed as liquid 19 at the top of the column. Nitrogen enriched gas is condensed in condenser 11 by heat exchange with expanded oxygen enriched liquid 12 (rich liquid) removed from the bottom of the column. The vaporized rich liquid 15 is warmed in the heat exchanger, expanded in turbine 7 to provide refrigeration for the process and is removed as waste after further warming. Gaseous nitrogen 17 is removed from the top of the column and is warmed in the heat exchanger.
U.S. Pat. No. 5,144,809 describes a process for nitrogen production wherein the column and exchangers are combined into a single plate fin exchanger. A portion of the medium air stream is subjected to dephlegmation to yield medium purity N2 (98-99%). This process provides low cost equipment but is limited to applications where the required purity is not stringent. Its power consumption is relatively high.
U.S. Pat. No. 4,867,773 and U.S. Pat. No. 4,966,002 describe a process similar to the classical process but a portion of the vaporized rich liquid extracted at the bottom of the distillation column is recompressed and recycled back to the distillation column or to the air stream feeding the distillation column. This arrangement allows some improvement over the classical process in terms of power consumption.
U.S. Pat. No. 4,848,996 adds a short column above the rich liquid vaporizer of the U.S. Pat. Nos. 4,867,773/4,966,002 process to yield a gaseous stream with similar composition to air (synthetic air). This stream is then recycled back to the air stream at an interstage of the air compressor to eliminate a separate recycle compressor.
U.S. Pat. No. 4,883,519 describes an improvement over the U.S. Pat. Nos. 4,867,773/4,966,002 process by partially vaporizing the rich liquid, recycling the resulting gaseous stream and expanding to lower pressure and vaporizing it in another exchanger.
U.S. Pat. No. 4,927,441 describes an improvement process over the U.S. Pat. No. 4,883,519 process by adding a short distillation column and distilling the bottom rich liquid of the high pressure column into a gaseous stream at lower pressure having a composition similar to air and a second liquid stream. The new gaseous "air" stream is recycled to an interstage of the main air compressor and recombined with the main air stream feeding the distillation column. This distillation column separates the feed into a nitrogen product stream at the top and a bottom rich liquid (rich in O.sub.2). The second liquid stream is expanded to lower pressure and subsequently vaporized to yield the waste nitrogen stream. A portion of gaseous nitrogen stream at the top of the column is split into two portions: The first portion is condensed in an exchanger located at the bottom of the short column to provide necessary reboil for this column. The second portion of gaseous nitrogen is condensed in another exchanger to provide the required duty for the vaporization of second liquid stream.
As previously mentioned and illustrated in the above description of the evolution of the process cycle, an improvement of the efficiency of the process results in an additional complexity of the process and consequently an increase in capital cost.